Because he didn't have a ST, and his k rate is down and his walk rate is up.
I haven't watched him pitch, but his numbers suggest he's been pretty lucky so far this year.
He's taken the ball at least 2 times, and perhaps as many as 4 times, when the team needed him to go 6 or more innings after the bullpen was wrecked ahead of him. He took the hit to his ERA as he tired later in the game, but he kept pitching and getting guys to put the ball in play to his defenders.
For what it's worth, with Riley on the IL, Duvall/Inciarte/Acuna across the outfield is a very, very good defensive outfield, and Keuchel has traditionally been a guy to let his defense back him up.